Stereo sound playback is commonly used in entertainment systems. It reproduces sound using two or more independent audio channels to create an impression of sound heard from various directions, as with natural hearing. Stereo sound is preferably played through a pair of stereo speakers that are located symmetrically with respect to the user. However, asymmetrical or unbalanced stereo speakers are inevitably encountered in reality. Examples include the stereophonic configuration in cars relative to the driver position and the unbalanced speaker setup on small-scale mobile devices. Asymmetric loudspeaker setups do not create good spatial effects. This is because the stereo image collapses if the listener is out of the sweet spot. In response, many sound images are localized at the position of the closest loudspeaker. This results in narrow soundfield distribution and poor spatial effects.
One common example of an asymmetric speaker arrangement occurs in mobile devices such as smartphones. It is getting more and more popular to equip mobile devices with stereo speakers. However, it is difficult to embed a pair of symmetrical speakers due to hardware constraints (e.g., size, battery), especially for smart phones. One solution is to use the embedded ear-piece receiver as a speaker unit. However, the frequency responses of the receiver and speaker are inevitably different (e.g. due to different baffle sizes), which leads to poor stereo effects and an unbalanced stereo sound image. Equalization of the receiver/speaker responses can address the unbalanced stereo sound image, but it does not achieve sound stage widening.
One option for creating a widened sound stage is to implement virtual source rendering with cross talk cancellation. Previous research explores the possibility of virtual source rendering using an ‘irregular’ loudspeaker arrangement (see e.g. “360 localisation via 4.x RACE processing” by Glasgel, 123rd AES Convention and “Experiments on the synthesis of virtual acoustic sources in automotive interiors” by Kahana et al, 16th International Conference, Spatial Sound Reproduction). This research is limited to the rendering of a single virtual source. Optimisation for a balanced stereo stage is not considered. Additionally, both methods only consider cases with geometrical asymmetry; they fail to mitigate discrepancies that are due to other asymmetries, such as differences in the natural frequency responses of the two speakers. These methods are thus incapable of optimising the asymmetrical speaker setup on smart phones. They also suffer from poor playback quality (including significant pre-echoes in filter design) and the robustness of soundfield widening effect is limited, especially in difficult car environments.
It is an object of the disclosure to provide concepts for improving the playback of audio signals through unbalanced speaker set ups.